Question Loud bang when PSU plugged in, but it works fine ?

[BurningSoul]

Hi guys,

I have donated my 6 year old computer to my partner, who now plugs it into a smart switch a uses that to switch off the mains power to it when not using it.

However, when the PC has been disconnected from mains, and mains is turned back on it makes a loud bang. This only happens when it has been disconnected for a longer period of time, cca 1+ days. When I disconnect it from mains for say 1 hr, the bang does not happen.

The PC works perfectly fine apart from this bang, no smoky smells either or visible damage, before I donated it it also worked fine but I never turned mains off to it.
I am no expert in electrics, but it doesn't like a PSU problem to me, more like something to do with the current? Any thoughts?

The PC has a Corsair RM650, GTX 1080, Ryzen 3700, 16 GB RAM and 3 x HDD.
Thanks

 

[Ralston18]

Why is the computer being turned off at the mains?

The normal process is to use the Windows shutdown icon and let Windows finish up its' housekeeping and preparation for the next boot.

That said: the specific source or location of the bang needs to be discovered. It could be in the PSU or the source could be somewhere else within the system.

As a general thought, one possibility is expansion/contraction. If the computer is turned off and then on again while warm or hot then no problem

However, if the computer is left to cool down then turning the computer on may immediately heat something up which expands and then buckles or moves in some manner that generates the noise.

Start by powering down the system, unplugging, and opening the case.

Look for signs of damage, anything loose, bent, cracked, touching, etc..

You may only be able to discover the source location by leaving the case open and restarting the computer when cold.

That must be done very carefully and if you are not comfortable doing so then get help from a knowledgeable person who can help.

 

[Misgar]

Would you describe this bang as a loud "crack" (snapping sound)?

I frequently hear these noises when plugging laptop power supplies into live mains outlets, or inserting a live IEC lead into the back of an ATX computer PSU.

The noise is caused by a high current arc forming briefly between two electrical contacts, e.g. the tip of the mains plug and the contact in the socket, or inside the mains on/off switch itself.

At the instant when you apply mains power to a switched mode power supply, a short duration current surge, typically 20A to 100A, flows into the bulk capacitor as the PSU charges up to 340V DC (from a 220V AC supply).

This audible crack is often accompanied by a brief flash of white light, if you perform the connection in a darkened room.

Any other loud "bang" is not normal and could indicate a fault has occured.

Good quality ATX PSUs have a thermistor with bypass relay to reduce the intensity of this switch on surge. Poor quality PSUs may lack even a basic thermistor and have larger switch on currents.

Is the RM650 being used on a different mains outlet or in another room? Did you use the smart switch on your PC? Does the bang come from inside the computer, e.g. the PSU, or somewhere outside the computer, e.g. from the smart switch?

I'd be inclined to disconnect the smart switch and plug the PC directly to the wall outlet. The smart switch may not "like" the high current spikes into the RM650.

Do not be tempted to remove the lid and explore inside the power supply. It contains a number of Safety Critical Components which should only be replaced with identical parts and of course, there may still be LETHAL voltages inside after it is switched off.

https://blog.clevercompliance.io/electrical-product-compliance/safety-critical-components/